I never previously measured the performance of the oven (temperature wise) but I never had any problems with my baking, like I do now.
I have the Taylor Oven Thermometer now and I put it in the oven middle vertically and horizontally. I put the oven on Bake @ 350F. 10 min after the oven had met the temperature (supposedly) the temperature was at 225F. Wow. I knew it could be off, but by that much??
So, I put it on Convection @350 and the oven heats up properly. 5 min after it meet the temperature, it is still at 350F and 15 min later it does not have spikes more than 10-20F. I then change to Bake and it keeps the temperature at 350F 20 min later.
I know I can change the calibration of the oven by 35F, but that would not help me - the food would burn when I would use convection.
I think that the bake element is working - it got hot when the oven was pre-heating.
I don't think the temperature probe is bad, or is it?
I never got any error codes in the system either. I have the service manual, as well, but it does not state how the heating process works (although it does give the relays and connections info: http://ww2.justanswer.com/uploads/smitty1486/2011-10-05_004525_jt950s0a3ss.pdf